Three stage power source for electric ARC welding

ABSTRACT

A three stage power source for an electric arc welding process comprising an input stage having an AC input and a first DC output signal; a second stage in the form of an unregulated DC to DC converter having an input connected to the first DC output signal, a network of switches switched at a high frequency with a given duty cycle to convert the input into a first internal AC signal, an isolation transformer with a primary winding driven by the first internal high frequency AC signal and a secondary winding for creating a second internal high frequency AC signal and a rectifier to convert the second internal AC signal into a second DC output signal of the second stage, with a magnitude related to the duty cycle of the switches; and, a third stage to convert the second DC output signal to a welding output for welding wherein the input stage has a regulated DC to DC converter with a boost power switch having an active soft switching circuit.

The invention relates to the field of electric arc welding and moreparticularly to an improved three stage power source for such weldingand a novel relationship between the first two stages of the three stagepower source.

INCORPORATION BY REFERENCE AND BACKGROUND OF INVENTION

Electric arc welding involves the passing of an AC or DC current betweena metal electrode and a workpiece where the metal electrode is normallya cored metal wire or solid metal wire. A power source is used to createa given current pattern and/or polarity between the advancing electrodewire and workpiece so that the arc will melt the end of the advancingwelding wire and deposit the molten metal on the workpiece. Althoughvarious converter technologies are used for power sources, the mosteffective is an inverter based power source where a switching networkincludes switches operated at high frequency to create the desiredwaveform or current level for the welding process. An inverter typepower source is discussed in Blankenship U.S. Pat. No. 5,278,390 wherethe inverter is controlled by “waveform control technology” pioneered byThe Lincoln Electric Company of Cleveland, Ohio. The actual waveform isgenerated by a series of short pulses created at a frequency generallyabove 18 kHz and the group of short pulses has a profile controlled by awaveform generator. In accordance with standard power source technology,the input signal to the inverter stage of the power source is rectifiedcurrent from a sine wave power supply. An appropriate power factorcorrecting converter is common practice and is either a part of theinverter switching network itself, as shown in Kooken U.S. Pat. No.5,991,169, or is located before the inverter stage, as shown in ChurchU.S. Pat. No. 6,177,645. Indeed, a power source with a power factorcorrecting converter or stage has been known in the welding art for manyyears. Another power source employing an input power factor correctingconverter in the form of a boost converter is shown in Church U.S. Pat.No. 6,504,132. The two patents by Church and the patent by Kooken areincorporated by reference herein as background information. In bothKooken U.S. Pat. No. 5,991,169 and Church U.S. Pat. No. 6,504,132 theactual welding current is regulated by an output chopper or buckconverter and isolation is obtained by a transformer either in theoutput of the inverter stage or in the output of the input boostconverter. These various topologies for power sources are commonknowledge in arc welding technology. In these prior art patents, theactual welding current, voltage or power is regulated in or before theoutput stage of the power source, which output stage is either aninverter or a chopper. Neither the inverter, nor the chopper isunregulated to produce a fixed, lower voltage DC bus for driving aregulated welding stage.

Isolation of the welding operation is a characteristic of most powersupplies for welding. The term “welding” includes “plasma cutting.” InVogel U.S. Pat. No. 5,991,180, a preregulator using a boost converter isdirected to a converter which is disclosed as a chopper having an outputisolation transformer located after welding regulation and directlydriving the welding operation. In this power source, the chopper networkis controlled to create the desired regulated output welding current andisolation is provided in the output stage. In a like manner, ThommesU.S. Pat. No. 5,601,741 discloses a boost converter for driving a pulsewidth modulated controlled inverter providing the regulated outputsignal to the actual welding operation. In both Vogel and Thommes, thesecond stage is regulated to direct the power factor controlled currentfrom a preregulator into a welding operation. Welding regulation is inthe second stage and is normally driven by a pulse width modulatorcontrol circuit. Both Vogel and Thommes are incorporated by referenceherein as background technology. In Moriguchi U.S. Pat. No. 6,278,080 aninverter type power source is regulated to control the desired weldingcurrent. Isolation is obtained by a transformer between the controlledsecond stage inverter and the welding output which is disclosed as a DCwelding operation. A similar power source is shown in Moriguchi U.S.Pat. No. 5,926,381 and Moriguchi U.S. Pat. No. 6,069,811 wherein theisolation of the control current from the inverter stage is at theoutput of the inverter and directly drives the welding operation.Moriguchi U.S. Pat. No. 5,926,381 discloses the common arrangement forusing the voltage at the output of the first stage boost converter toprovide the controller voltage for either the regulated inverter stageor the boost converter itself. The three Moriguchi patents areincorporated by reference herein as background information showing theprior art power source where a regulated inverter is driven by an inputboost converter or a DC output of a rectifier to produce a controlledwelding current directed to an output transformer used for isolation.The secondary AC signal of the isolation transformer is directly usedfor the welding operation. There is no third stage topology as used inthe novel power source of the invention.

Turning now to non-welding technology, an aspect of the invention is theuse of a synchronous rectifier device at the output of a DC/DC secondstage converter. Synchronous rectifiers are common practice and one suchrectifier is illustrated in Boylan U.S. Pat. No. 6,618,274. Calkin U.S.Pat. No. 3,737,755, discloses a DC/DC converter for low power use wherea fixed regulated current is directed to a non-regulated inverter toprovide a non variable output DC signal. Any control of thenon-regulated inverter is at the input side of the inverter so that theinput DC signal is the only parameter that can be regulated to controlthe fixed output DC signal of the inverter. This is a topography thatrequires a control of the signal to the inverter so that the inverterprovides a controlled fixed output signal. The non-welding generalbackground technology in Boylan and Calkin is incorporated by referenceherein to show a synchronous rectifier and a version of a non-regulatedinverter where any regulation is performed before the inverter bycontrolling the level of the input DC signal. Neither of these patentsrelate to a power source for welding and are only incorporated byreference as general technical concepts, such as synchronous rectifierdevices and unregulated inverters. A non-welding two stage AC to DCconverter is shown in Smolenski U.S. Pat. No. 5,019,952 for impartingminimum harmonic distortion to the current flowing into the converter.The load is not variable and does not require regulation as demanded ina welding operation. This patent is incorporated by reference to showgeneral technology not related in any way to the demands of a powersource for electric arc welding.

These patents constitute the background information relating to a powersource that must be regulated by a welding operation where suchregulation is by a feedback loop of average current, average voltage,and power of the actual welding operation. Fixed load power sources arenot relevant to the invention, except as general technical information.

In the past, an inverter in a power source outputted a welding currentregulated by a parameter in the welding operation, such as current,voltage or power. This inverter was normally controlled by a pulse widthmodulator wherein the duty cycle of the switches operated at highfrequency was controlled by the feedback from the welding operation sothat the duty cycle was adjusted in a range substantially less than100%. This type of PWM controlled inverter is referred to as a regulatedsingle stage inverter. Such inverter formed the output of the powersource and was the last stage of the power source. Lower duty cyclesresulted in higher primary currents and more losses. The efficiency ofthe inverter varied according to the duty cycle adjustment caused by therequirement of regulating the output of the single stage inverter tocreate an output signal suitable for welding. Using a power source wherethe final stage is a regulated single stage inverter resulted in heatlosses, lower efficiency, high cost and increased component size. Forthese reasons, some welding source manufacturers have marketed powersources as being better than an inverter power source because they donot use inverters with the resulting high cost and other difficulties.An inverter stage which had the dual function of isolating the outputand regulating the current for the purposes of creating a currentsuitable for welding was to be avoided. See Hoverson U.S. Pat. No.6,723,957, incorporated by reference herein as background.

The Three Stage Power Source Improved by the Present Invention

The present invention is used with a three stage power source forelectric arc welding (plasma cutting) wherein the inverter of the powersource is a second stage as in the past, but is unregulated so that athird stage can be added to provide the actual regulation for creating acurrent suitable for welding. By using this novel three stage concept,the inverter can operate at a very high frequency of switching whereasthe output third stage can be a chopper operated at a lower frequency ofswitching. Consequently, the switching frequency is optimized by thefunction performed by the stage as opposed to the need for using highfrequency in a pulse width modulated inverter stage used for actualregulation of the output welding current. Furthermore, the isolated,fixed DC voltage to the regulated third stage can be substantially lowerthan the DC voltage from the input converter stage and much higher thanthe actual welding output voltage.

The three stage power source using the invention involves a noveltopography for a power source wherein the pulse width modulated inverteris merely a second stage for creating an isolated fixed output DC buswithout a feedback signal to the second stage pulse width modulatedinverter. This isolated bus is used in a third stage regulated by theactual welding parameters to create a current suitable for welding.Consequently, the invention involves an unregulated second stage notonly providing necessary isolation but also to producing a fixed DCoutput bus to be used by a third stage wherein welding regulation isaccomplished. The unregulated second stage inverter is operated at avery high frequency with a duty cycle that is fixed during operation ofthe power source. The frequency is over 18 kHz and preferably about 100kHz. The duty cycle is fixed at various levels; however, the preferredduty cycle is close to 100% to give the maximum efficiency level. Theuse of a fixed, high duty cycle minimizes the current circulation timeof the phase shift modulator controlled inverter second stage tosubstantially reduce heat and increase efficiency. The output of thesecond unregulated inverter stage can be a rectifier using well knownsynchronous rectifier devices, which devices are controlled by thesecondary winding of the internal isolation transformer of the secondstage unregulated inverter. By using synchronous rectifier devices atthe output of the second stage, there is further improvement in thetotal efficiency of the power source. The first stage is either an inputrectifier or an input rectifier with a power factor correctingconverter. A first stage power factor correcting converter is preferred.This converter is after a standard rectifier or can be combined with therectifier. Of course, this converter can be a passive power factorcorrecting converter or an active converter such as a boost, buck orbuck+boost converter. The first stage of the invention produces a firstDC bus with a fixed voltage. By using a standard first stage for thepower source, the first DC output signal which is the input DC bus tothe unregulated inverter can be regulated and fixed at a value of about400-900 volts DC. The output of the unregulated, isolation inverterforming the second stage of the novel power source is a fixed DC bushaving a fixed relationship with the input DC bus from the first stage.The voltage of the second DC bus or output is substantially less thanthe voltage of the DC bus from the first stage. The power source thusproduces a second DC bus which has a fixed mathematical relationshipwith the input DC bus from the power factor correcting converter. Inaccordance with standard practice, the second stage unregulated inverterincludes an isolation transformer having a primary winding and asecondary winding so that the secondary winding is isolated from theinput of the power source. See Steiger U.S. Pat. No. 4,864,479,incorporated by reference herein. The unregulated, second stage invertercan be operated at a switching frequency to optimize the operation ofthe second stage inverter. Thus, extremely high switching frequency isused to reduce the size and cost of the components in the novel,unregulated second stage inverter. By utilizing a fixed duty cycle withphase shift control, voltage and current surges in the switching devicesare reduced to provide a soft switching operation. Indeed, in thepreferred embodiment, the duty cycle is fixed at close to 100% so thatthe switches are full on or full off. This drastically reduces thecirculated current in the second stage and greatly improves theoperating characteristics of the second stage inverter which alsoprovides the function of isolating the welding output of the powersource from the AC input of the power source. By having the switchingdevices in the second stage unregulated inverter operated at full on,this inverter has a high efficiency and is very flexible in operation.An isolation transformer determines the relationship between the fixedDC bus at the input side of the unregulated second stage (a “first DCoutput signal” from the first stage) and the DC output bus at the outputof this second stage (a “second DC output signal”). In some prior artpower sources, the duty cycle at the primary winding of the isolationtransformer in the regulated inverter is regulated by the weldingoperation. There is no regulation by the welding operation in either thefirst stage or second stage of the novel three stage power source towhich the present invention is directed.

A power source for electric arc welding having an active power factorcorrecting feature and tight output control of the energy directed tothe welding operation requires at least two switching stages. These twostages assure that instantaneous energy transferred into the powersource and transferred out the power source can be regulatedindependently with appropriate energy storage components. Thus, a powerfactor correcting power source for electric arc welding generallyrequires two independent switching control circuits. One of the controlcircuits is used to control the energy or the output current for thewelding operation. The other control circuit is used to control the DCsignal from the active power factor correcting converter forming thefirst stage of the power source. Thus, electric arc welding powersources having power factor correcting capabilities requires twoswitching networks each of which has independent control requirements.The first switching control is for the output welding current and theother switching control is for power factor correcting at the inputstage of the power source. This second switching control assures thatthe output of the first stage is a fixed DC voltage referred to as a “DCbus.” The voltage of the DC bus itself is used to control the firststage converter to assure that the DC bus from this converter has afixed voltage level. To recapitulate an inverter based power source forelectric arc welding requires two separate switching networks and twocontrol circuits for these networks.

An inverter based power source for electric arc welding has anotherconceptual requirement. One of the stages in the power source mustprovide electrical isolation between the variable input AC signal andthe regulated output current suitable for welding. The isolation deviceis normally in the form of a transformer. In prior art, two stageinverter based power sources there are two locations for the isolationdevice. In the first example, the power factor correcting input stage isnot isolated and an isolation transformer is provided in the secondstage regulated output inverter. In another example, isolation is in thefirst stage power factor correcting converter. In this second example, anon-isolation output inverter or other non-isolation converter can beused as the second stage. The first example is more efficient than thesecond example due to 60 Hz effect on the RMS current at the input sideof the power source. In recapitulation, the second conceptualrequirement of a welding power source is isolation.

The two requirements of an active power factor correcting power sourcefor welding are (a) two separate and independent control circuits fortwo separate switching networks and (b) an appropriate structure forisolating the input of the power source from the output of the powersource. These basic requirements of inverter based power sources areimplemented in the background three stage power source. The unregulatedsecond stage is an isolation stage between two regulated non-isolationstages to form a unique arrangement involving a three stage inverterbased power source. The novel three stage power source is more efficientthan the two stage inverter based power source assuming the same powerfactor correcting preregulator is used. Thus, the novel three stagepower source is more efficient, but still has the essentialcharacteristics required for a power source used in electric arcwelding. There are two independently controlled switching networks.There is an isolation stage. These constraints are accomplished in amanner to increase efficiency and obtain better welding performance andbetter heat distribution of the power switching components.

Since the second unregulated inverter stage of the three stage powersource provides system isolation, many types of non-isolated converterscan be used as the power factor correcting preregulator. A boostconverter is the most popular converter due to the current shapingfunction and the continuous line current characteristics of this type ofconversion. However, the output voltage of the boost converter is higherthan the peak of the highest line voltage, which peak can be as high as775 volts. Thus, other active power factor correcting regulators can beused with the invention, which is a three stage power source wherein thesecond stage is unregulated and provides isolation. One of the otheroptions for the active power factor correcting input or first stage is astep-up/step-down converter so that the primary voltage bus or input busto the second stage can be lower than the peak of the input AC voltagesignal to the power source. This type of power factor correctingconverter still produces low harmonics. One such power factor converteris referred to as a buck+boost converter. A 400 volt to 500 volt DC busused for the second stage is obtained with an input AC voltage in therange of 115 volts to 575 volts. Irrespective of the AC voltage to thefirst stage, the output voltage of the active power factor converter iscontrolled to be at a level between 400 volts and 500 volts. Other typesof active and passive power factor correcting inverters can be used inthe invention. The preferred converter is active thus constituting asecond switching network requiring a second control circuit. When usingthe term electric arc welding, it also includes other output processes,such as plasma cutting.

As so far explained, the three stage power source using the inventioninvolves a three stage power source for electric arc welding. A feedbackcontrol in the third stage creates an output current suitable forwelding. The input first stage is normally an active power factorcorrecting converter requiring a second switching network and a secondindependent control circuit. This three stage topography is not used inthe prior art. By having this topography, the added second stage ismerely used to convert the high voltage DC bus at the primary side ofthe second stage to a lower voltage DC bus at the secondary side of thesecond stage isolated from the primary side. Thus, the three stageinvolves a DC bus at the secondary side of the second stage so that thebus can be used for regulation of welding power. The term “bus” means aDC signal that has a controlled fixed level. The three stage powersource has a first DC bus from the input stage called the “first DCoutput” which first DC output has a controlled DC voltage. There is asecond DC bus at the secondary side of the second stage called the“second DC output” which second DC output is also a controlled DCvoltage level. The creation of a second DC bus at the secondary side ofan unregulated inverter has advantages, other than the advantagesassociated with the use of the unregulated second stage inverter as sofar described. The secondary DC bus or second DC output is isolated fromthe primary side of the second stage so that there is no isolationrequired in the third stage welding control circuit. In other words, theoutput control circuit, such as a chopper, has an input DC bus with afixed voltage level. In practice, the chopper has a controller with acontrol voltage that is derived from the input DC to the chopper. Thisinput DC signal is isolated from the input power. Consequently, thecontrol voltage for the controller of the output stage or chopper can bederived from a non-isolated DC source. This is normally the input signalto the chopper. Separate isolation of the control voltage for thecontroller used in the output stage is not required. The use of a fixedDC bus from the second stage allows the DC voltage to the output thirdstage, which is regulated by the welding operation, to be much lowerthan the normal input primary DC bus (“first DC output”) of the powersource. In the past, the output of the power factor converter is arelatively high level DC signal based upon the use of a boost converter.This high DC voltage was directed to the regulated inverter stage foruse in outputting a current suitable for the welding. By using thepresent invention the high voltage from the output bus of the powerfactor converter is drastically reduced. It is more efficient to converta 100 volt DC bus into a 15 volt control power than to convert a 400volt DC bus to a 15 volt control power.

A second stage of the background three stage power source is in the formof an unregulated DC to DC converter has an input connected to the firstDC output signal and an output in the form of a second DC output signalelectrically isolated from the first DC output signal with a magnitudeof a given ratio to the first DC output signal. The power sourceincludes a third stage to convert the second DC output signal to awelding current for the welding process. The third stage of the powersource includes a regulated converter such as a chopper or inverter.When using an inverter, the output is a DC signal directed to a polaritynetwork or switch, which switch allows DC welding by the power source.The polarity switch allows welding either DC negative, DC positive orAC. The welding process, using either a chopper or an inverter, can beperformed with shielding gas, such as MIG welding, and can use any typeof electrode, such as tungsten, cored wire or solid metal wire. Inaccordance with an aspect of the invention, the output of theunregulated DC to DC converter is substantially less than the input tothe second stage. In most instances, the input and output of the secondstage are DC voltages with generally fixed magnitudes.

THE INVENTION

There are several benefits to operating welding inverters with highswitching speeds. For instance, smaller magnetics translate intoimproved portability. Another advantage is the potential to have ahigher band width control system, which system will result in a betterarc performance. Thus, the novel three stage power source describedabove is improved by the present invention. This background three stagepower source has power switches operated at extremely high switchingspeed, exceeding 18 kHz. The boost power switch for the first stage andthe four power switches for the unregulated second stage are alloperated at high frequency to obtain the benefit of high switchingspeed. There is a downside to the use of such higher switching speeds.Such switching speeds cause switching losses. If the switching lossesare not reduced the power source efficiency and reliability aredecreased. The switching losses are caused by the overlap of current andvoltage during switching, either from the on condition to the offcondition or from the off condition to the on condition. To reduce theswitching losses, either the voltage or the current must be held nearzero during the switching. Switching transition can be either zerovoltage or zero current or both. This is called “soft switching.” Whatare termed resonant or quasi resonant techniques have heretofore beenused to obtain soft switching by zero voltage or zero current at highswitching speeds. However, this type of prior soft switching controloften causes higher current and voltage stresses because of thesinusoidal waveforms and still has conduction losses. However, there areprior soft switching circuits that employ zero voltage transitionconverters or zero current transition converters in a manner to reduceboth the switching losses and the conduction losses.

It is known that the unregulated second stage inverter of the novelthree stage power source to which the present invention is directed usesa phase shift PWM to control the output power. By fixing the phase shiftat a high level near 100%, preferably above 80%, the switching losses inthe second unregulated stage are limited. By using a fixed phase shiftPWM control the second stage is operated near full conduction to producelow conduction losses. The second unregulated stage is soft switchedinherently. In accordance with the invention, the three stage powersource described above has soft switching in the input stage. To thisend, the present invention involves the use of an active soft switchingcircuit for the first input stage to be combined with the inherent softswitching of the second unregulated stage. This combination of addedsoft switching with inherent soft switching has substantially increasedthe efficiency of the novel three stage power source to which theinvention is directed.

The active soft switching circuit of the first stage is the type circuitdescribed in a 1991 article by the IEEE entitled High Efficiency TelecomRectifier using A Novel Soft-Switching Boost-based Input Current Shaper.This November 1991 article is incorporated by reference herein. Thistype circuit is also described in a 2002 article entitled A New ZVT-PWMDC-DC Converter by the IEEE. This article from the IEEE Transaction onPower Electronics is dated January 2002 and is incorporated by referenceherein. Another active circuit for soft switching is a voltagetransition-current transition circuit described in a 2004 articleentitled A New ZVT-ZCT-PWM DC-DC Converter published by IEEETransactions on Power Electronics published in May, 2004. This articleis also incorporated by reference herein. These articles describe anactive soft switching circuit or circuits of the type used in the firststage of a three stage power source. The invention combines an activesoft switching for the first input stage and an inherently soft switchedunregulated inverter using a phase shift PWM control. Steigerweld U.S.Pat. No. 4,864,479 is incorporated by reference herein to show a commonunregulated inverter using phase shift control. This type of unregulatedpower stage has a topography that increases the efficiency by minimizingthe circulating currents through the use of a fixed high duty cycleswitching operation. The unregulated inverter operated at a fixed dutycycle will achieve soft switching on all the primary switches with aminimum amount of conduction losses. This concept is used in the secondstage of the three stage power source to which the invention isdirected.

In accordance with the invention, the high switching speed power switchof the first stage of a three stage power source is soft switched withan active circuit to reduce both the losses of the switch and the lossesof the output rectifier. Furthermore, the soft switches input stage iscombined with a second stage having an inherent soft switchingcapability using a fixed duty cycle, phase shift unregulated inverter.The combination of an active soft switching circuit for the first stagecombined with the inherent soft switching of a fixed duty cycleunregulated inverter substantially increases the efficiency of a threestage power source of the novel type to which the present invention isdirected.

By using an active soft switching circuit on the first input stage ofthe three stage power source, the pulse width modulator converter of thefirst stage has zero voltage switching for the active converter switchand zero reverse recovery current for the output diode. This softswitching is without increasing voltage or current stresses, i.e.conduction losses of the two components. This soft switching circuit forthe power switch (active) for the first stage includes a zero voltagetransition using a network with an inductance branch and capacitorbranch both in parallel with both the active pulse width modulatingpower boost switch and the passive output switch or output boost diode.The two branch network includes an induction branch capacitance branchcontrolled by switching of an auxiliary switch. The auxiliary switch isalso connected in parallel with the pulse width modulated power boostswitch and is turned on for a short interval just prior to the turn onof the pulse width modulated switch. The network inductor current rampsup until it turns off the output rectified diode, communicating it witha soft switching operation. The inductor current continues to increasebringing the voltage across the pulse width modulated circuit to zero ata time prior to the turn on of the boost switch. An anti-parallel diodeof the pulse width modulator switch is thus forward biased. The turn onsignal for the power switch is applied while the anti-parallel diode isconducting to provide a zero voltage switching of the modulating switchat turn on. The auxiliary switch is then turned off and the modulatingpower switch is turned on. The auxiliary diode and capacitor provides asnubber to the voltage across the auxiliary switch so that the auxiliaryswitch is not stressed at turn off. The inductor branch current rapidlydrops to zero, at which time the auxiliary switch turns off. Theremainder of the operation is the same as that of a conventional pulsewidth modulated boost converter, except the energy stored in the twobranch network is transferred to the load when the main switch is turnedoff. In some descriptions of these two branches, they are referred to asa resonant circuit which may be technically true, but not necessary tothe soft switching function.

The auxiliary switch controlled two branch circuit is used in the firststage of the present invention to provide soft switching of both thepower switch and the output diode. Such a circuit is described in HuaU.S. Pat. No. 5,418,704, incorporated by reference herein. Softswitching of the first stage and the natural soft switching of thesecond stage is the result of using the present invention.

In accordance with the present invention there is provided a three phasepower source for an electric arc welding process. This power sourcecomprises an input stage having an AC input and a first DC outputsignal, a second stage in the form of an unregulated DC to DC converterhaving an input connected to the first DC output signal, a network ofswitches switched at a high frequency with a given duty cycle to convertthe input signal into a first internal AC signal, and isolationtransformer with a primary winding driven by the first internal highfrequency AC signal and a secondary winding for creating a secondinternal high frequency AC signal and a rectifier to convert the secondinternal AC signal into a DC output signal of the second stage. Themagnitude of the output signal for the second stage is related to thefixed amount of overlap between the phase shifted switches, which usephase shift controlled by a pulse width modulator so that the secondstage is inherently soft switched. A third stage in the power source isused to convert the second DC output signal from the second stage to awelding output for the welding process. This three stage power source isimproved by providing a DC to DC converter in the first stage, where theconverter has a power switch with a soft switching circuit. Thus, a softswitching circuit to the first stage compliments the inherent softswitching of the phase shift, unregulated second stage to increase theefficiency of the first two stages in the three stage power source.

In accordance with another aspect of the present invention, the softswitching circuit of the first input stage of the three stage powersource is an active snubber circuit with an auxiliary switch operated inunison with the power switch to positively drive the voltage toward zeroduring both switching transitions. The DC to DC converter of the firststage has an output or boost diode which is also soft switched by thefirst stage soft switching circuit. In accordance with another aspect ofthe invention, the DC to DC converter of the first stage has a positiveand a negative output lead with a capacitor joining the leads and adiode clamping the positive end of the auxiliary switch to the positiveoutput lead. The three stage power source with the unique combination ofan active soft switching on the first stage and an inherent softswitching on the second stage is used with a third stage chopper. In anoption, the output chopper has a soft switching circuit for its powerswitch. All of these features of the present invention improve a threestage power source having as is novel feature a center unregulated,isolation stage to increase the efficiency of the power source whilemaintaining the advantage of its three stage topography.

The present invention is the combination of an input stage and anunregulated center stage of a three stage power source, wherein thefirst stage has an active soft switching circuit for the boost powerswitch and an inherent soft switching for the phase shifted unregulatedsecond stage. Consequently, the invention involves a two stage AC to DCconverter comprising an input stage having an AC input and a first DCoutput signal and a second stage. The second stage is in the form of anunregulated DC to DC converter having an input connected to the first DCoutput signal, a network of switches switched at a high frequency with agiven duty cycle to convert the input into a first internal AC signal,an isolation transformer with a primary winding driven by the firstinternal high frequency AC signal and a second winding for creating asecond internal AC signal and a rectifier to convert the second internalAC signal into a second DC output signal of the second stage. Themagnitude of the output signal for the second stage is related to theamount of overlap between the phase shifted switches. The input stageincludes a power switch having a soft switching network which network isan active snubber circuit with an auxiliary switch operated in unisonwith the power switch of the first stage.

The primary object of the present invention is the provision of a novelthree stage power source wherein the first stage has an active softswitching circuit for the fast switching power switch and the secondstage is an unregulated inverter forming a part of an isolation stage,which inverter has a soft switching characteristic based upon a fixedhigh duty cycle for its several switches.

Another object of the present invention is the provision of a two stageinverter for use in power conversion, which converter includes a powerswitch with an active soft switching circuit and the second stageinvolves an unregulated inverter with a fixed duty cycle controlled byphase shift.

Yet another object of the present invention is the provision of a threestage power source, as defined above, which three stage power sourcealso includes an output stage in the form of a chopper with the powerswitch of the chopper having a passive soft switching circuit.

Yet a further object of the present invention is the provision of athree stage power source, as defined above, which power source includesan active soft switching circuit for the first stage, an inherent softswitching characteristic for the second stage, and a passive softswitching circuit for the third stage.

These and other objects and advantages will become apparent from thefollowing description taken together with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS Three Stage Power Source

FIG. 1 is a block diagram illustrating a three stage power source anddisclosing an embodiment of the three stage power source improved by theinvention;

FIG. 2 and FIG. 3 are block diagrams similar to FIG. 1 disclosingfurther embodiments of the three stage power source;

FIGS. 4-8 are partial block diagrams illustrating the three stage powersource with different first stage embodiments;

FIG. 9 is a block diagram showing the last two stages of the three stagepower source wherein the output stage provides AC welding current;

FIG. 9A is a block diagram of a waveform technology control circuit foruse in the three stage power source illustrated in FIG. 9, together withgraphs showing three welding waveforms;

FIG. 10 is a block diagram illustrating a second and third stage of thethree stage power source wherein the output stage is DC welding current;

FIG. 11 is a block diagram illustrating the topography of the threestage power source for creating current suitable for electric arcwelding with two separate controller control voltage supplies;

FIG. 12 is a block diagram illustrating a specific three stage powersource employing the topography to which the present invention isdirected;

FIGS. 13-16 are wiring diagrams illustrating four different circuits forcorrecting the power factor in the first stage of the three stage powersource;

FIG. 17 is a combined block diagram and wiring diagram illustrating thepreferred embodiment of the unregulated inverter constituting the novelsecond stage of the three stage power source to which the presentinvention is directed;

FIGS. 18-21 are wiring diagrams showing several inverters used as thesecond stage unregulated, isolation inverter comprising the novel aspectof the three stage power source to which the present invention isdirected;

The Invention

FIG. 22 is a wiring diagram of the first input stage and secondisolation stage forming an embodiment of the present invention;

FIG. 23 is a wiring diagram of a second embodiment of the presentinvention;

FIG. 24 is a wiring diagram illustrating a three stage power sourcewherein the output stage is a chopper with a passive soft switchingcircuit;

FIG. 25 is a wiring diagram showing the active soft switching circuitused in the embodiment of the invention shown in FIG. 22;

FIG. 26 is a wiring diagram showing the active soft switching circuitused in the preferred embodiment of the invention; and,

FIG. 27 is a graph of the voltage curves and trigger signals for themain power switch and the auxiliary switch of the circuit illustrated inFIG. 26.

THREE STAGE POWER SOURCE FIGS. 1-21

The present invention is a modification of a novel three stage powersource for use in electric arc welding as developed by The LincolnElectric Company, which power source is not prior art to the presentinvention. The new three stage power source has an input stage forconverting an AC signal into a first DC output bus. This output bus hasa fixed voltage level and is directed to the input of a second stagebest shown in FIG. 16. This novel second stage of the three stage powersource is an unregulated inverter which includes an isolation featureand has a second DC output or second DC bus which is proportional to theDC input bus. The level relationship is fixed by the construction of theunregulated inverter. The unregulated second stage inverter has aswitching network wherein the switches are operated at a high switchingfrequency greater than 18 kHz and preferably about 100 kHz. Theswitching frequency of the switch network in the unregulated inverterforming the second stage of the power source allows use of smallmagnetic components. The isolated DC output of the unregulated inverteris directed to a third stage of the power source. This third stage canbe either a chopper or inverter which is regulated by a weldingparameter, such as current, voltage or power of the welding operation.In the modification this third stage is preferably a chopper. Thetopography of the three stage power source has an input stage to producea first DC signal, a second unregulated DC to DC stage to provide anisolated fixed DC voltage or DC bus that is used by the third stage ofthe power source for regulating the current used in the weldingoperation. Three examples of a three stage power source to which thepresent invention is directed are illustrated in FIGS. 1-3. Power sourcePS1 in FIG. 1 includes first stage I, second stage II, and third stageIII. In this embodiment, stage I includes an AC to DC converter 10 forconverting AC input signal 12 into a first DC bus 14. The input 12 is anone phase or three phase AC line supply with voltage that can varybetween 200-700 volts. Converter 10 is illustrated as an unregulateddevice which can be in the form of a rectifier and filter network toproduce DC bus 14 identified as (DC#1). Since the AC input signal is aline voltage, DC bus 14 is generally uniform in magnitude. Unregulatedinverter A is a DC to DC converter with an isolation transformer toconvert the DC bus 14 (DC#1) into a second DC bus or second DC output 20(DC#2). Output 20 forms the power input to stage III which is converter30. The DC voltage on line 20 into a current suitable for welding atline B. A feedback control or regulation loop C senses a parameter inthe welding operation and regulates the current, voltage or power online B by regulation of converter 30. In practice, converter 30 is achopper, although use of an inverter is an alternative. By having athree stage power source PS1 as shown in FIG. 1, the switching networkof the second stage has a frequency that is normally higher than theswitching frequency of converter 30. Furthermore, the DC voltage in line20 (DC#2) is substantially less than the DC voltage from stage I on line14 (DC#1). In practice, there is an isolation transformer in inverter A.The transformer has an input or primary section or side withsubstantially more turns than the secondary section or side used tocreate the voltage on line 20. This turn ratio in practice is 4:1 sothat the voltage on line 20 is ¼ the voltage on line 14. For DC #1, thisvoltage is around 400 volts in practice.

The general topography of three stage power source to which the presentinvention is directed is illustrated in FIG. 1; however, FIG. 2illustrates the preferred implementation wherein power source PS2 hasessentially the same stage II and stage III as power source PS1;however, input stage I is an AC to DC converter 40 including a rectifierfollowed by a regulated DC to DC converter. The converted signal is a DCsignal in line 14 shown as a first DC bus (DC#1). The voltage on line 14is regulated as indicated by feedback line 42 in accordance withstandard technology. Thus, in power source PS2 the output weldingconverter 30 is regulated by feedback loop C. The voltage on line 14 isregulated by feedback loop shown as line 42. Since converter 40 is apower factor correcting converter it senses the voltage waveform asrepresented by line 44. By using power source PS2, the first DC bus 14is a fixed DC voltage with different one phase or three phase voltagesat input 12. Thus, output 20 is merely a conversion of the DC voltage online 14. DC#2 is a fixed voltage with a level determined by theisolation transformer and the fixed duty cycle of the switching networkin unregulated inverter A. This is the preferred implementation of thenovel power source employing three separate and distinct stages withstage II being an unregulated inverter for converting a fixed first DCoutput or DC bus to a second fixed DC output or DC bus used to drive aregulated welding converter, such as a chopper or inverter. As anotheralternative, stage I could be regulated by a feedback from the DC #2 busin line 20. This is represented by the dashed line 46 in FIG. 2.

Power source PS3 in FIG. 3 is another implementation of the three stagepower source. This is not the preferred implementation; however, thethree stage power source of the present invention can have the inputconverter 50 regulated by feedback loop 52 from the welding currentoutput B. With this use of a three stage power source, converter 50 isregulated by the welding output and not by the voltage on line 14 as inpower source PS2. With regulation from welding output B, converter 50 isboth a power factor correcting stage and a welding regulator. However,this implementation of the three stage power source is disclosed for acomplete technical disclosure.

As previously described, input stage I converts either a single phase ora three phase AC signal 12 into a fixed DC bus 14 (DC#1) for use by theunregulated inverter A constituting second stage II. The novel threestage power source generally employs a DC to DC converter in stage I toproduce the DC voltage indicated as line 14 in FIGS. 1-3. The DC to DCconverter of stage I can be selected to create the desired voltage online 12. Three of these converters are shown in FIGS. 4-6 wherein aninput rectifier 60 provides a DC voltage in lines 60 a, 60 b to a DC toDC converter which may be a boost converter 62, a buck converter 64 or abuck+boost converter 66, as shown in FIG. 4, FIG. 5 and FIG. 6,respectively. By using these converters, the DC to DC converter of stageI incorporates a power factor correcting chip, which chip allows thepower factor to be corrected thereby reducing the harmonic distortion atthe input of the power source. The use of a power factor correctinginput DC to DC converter is well known in the welding art and is used inmany prior art two stage topographies. Converters 62, 64 and 66preferably include a power factor correcting chip; however, this is notrequired. The main purpose of stage I is to provide a DC bus (DC#1) inline 12, which bus is indicated to be lines 14 a, 14 b in FIGS. 4-6 toproduce a fixed DC bus (DC#2) in line 12 indicated by lines 20 a, 20 bin the same figures. Power factor correction is not required to takeadvantage of the novel three stage topography. A non power factorcorrecting input stage is illustrated in FIG. 7 where the output lines60 a, 60 b of rectifier 60 are coupled by a large storage capacitor 68to produce a generally fixed voltage in lines 14 a, 14 b. Stage I inFIG. 7 does not incorporate a power factor correcting circuit or chip.However, the power source still involves three stages wherein the secondstage is unregulated isolated inverter A to produce a generally fixedvoltage on lines 20 a, 20 b. Another modification of input stage I isillustrated in FIG. 8 where a passive power factor correcting circuit 70is connected to a three phase AC input L1, L2 and L3 to produce agenerally fixed DC voltage across lines 14 a, 14 b, which linesconstitutes the DC bus 14 (DC#1) at the input of inverter A. Thedisclosures of modified stage I in FIGS. 4-8 are only representative innature and other input stages could be used with either single phase orthree phase input signal and with or without power factor correcting.

By providing low fixed voltage on output bus 20 illustrated as lines 20a, 20 b, the third stage of the novel three stage power source forwelding can be a chopper or other converter operated at a frequencygreater than 18 kHz. The switching frequencies of the unregulatedinverter and the regulated output converter may be different. Indeed,normally the switching frequency of the chopper is substantially lessthan the frequency of unregulated inverter A. Power source PS4 shown inFIG. 9 illustrates the use of the present invention wherein stage III isa standard regulated converter 100 of the type used for electric arcwelding. This converter is driven by fixed input DC bus 20 and isregulated by feedback from the welding operation 120 to provide currentsuitable for welding across output leads 102, 104. Leads 102 is apositive polarity lead and leads 104 is a negative polarity lead. Inaccordance with standard output technology for a two stage inverterbased power sources, leads 102, 104 are directed to a standard polarityswitch 110. This switch has a first position wherein lead 102 isdirected to the electrode of the welding operation 120 so the output ofpolarity switch 110 has a positive polarity on output line 110 a and anegative polarity on output line 110 b. This produces an electrodepositive DC welding process at weld operation 120. Reversal of polarityswitch network 110 can produce an electrode negative DC welding processat weld operation 120. Thus, a DC welding process with either DCnegative or DC positive can be performed according to the setting of thestandard polarity switch 110. In a like manner, polarity switch 110 canbe alternated between electrode negative and electrode positive toproduce an AC welding process at weld operation 120. This is standardtechnology wherein polarity switch 110 drives the DC output fromregulated converter 100 to produce either an AC welding process or a DCwelding process. This process is regulated and controlled by a feedbacksystem indicated as line or loop 122 directed to controller 130 forregulating converter 100 and for setting the polarity of switch 110 asindicated by lines 132, 134, respectively. By regulating the weldingoperation at stage III, the unregulated inverter at stage II can have arelatively higher switching frequency to reduce the component sizeswithin the second stage of the power source and can have close to 100%duty cycle switching to improve efficiency. The preferred embodiment ofthe three stage power source employs waveform control technologypioneered by The Lincoln Electric Company of Cleveland, Ohio. This typeof control system is well known and is schematically illustrated in FIG.9A wherein control circuit 150 processes a waveform profile as a voltageon line 152 a is outputted from waveform generator 152. The waveformprofile is controlled by feedback loop 122 as schematically illustratedby error amplifier 154 having an output 156. Thus, the profile of thewaveform from generator 152 is controlled by the feedback loop 122 andproduces a signal in output line 156. This line is directed to anappropriate pulse width modulator circuit 160 operated at a highfrequency determined by the output of oscillator 162. This frequency isgreater than 18 kHz and is often higher than 40 kHz. The regulatedconverter 100 preferably operates under 40 kHz. The output of the pulsewidth modulator, which is normally a digital circuit within controller130, is shown as line 132 for controlling the waveform by way ofregulated converter 100. In accordance with standard practice, thewaveform of inverter 100 can have any profile, either AC or DC. Thisfeature is schematically illustrated as waveform 152 b, 152 c and 152 dat the right portion of FIG. 9A. Waveform 152 b is an AC waveform of thetype used in AC MIG welding where a higher negative electrode amperageis provided. A higher positive amperage is also common. In waveform 152c, the amperage for both electrode negative and electrode positive isessentially the same with the length of the negative electrode portionbeing greater. Of course, a process for AC welding can be adjusted toprovide balanced AC waveforms or unbalanced AC waveforms, either infavor of electrode negative or electrode positive. When polarity switch110 is set for either a DC negative or a DC positive welding operation,a pulse welding waveform, shown as waveform 152 d, is controlled bywaveform generator 152. Various other waveforms, both AC and DC, can becontrolled by controller 130 so the welding operation 120 can beadjusted to be AC, or DC. Furthermore, the welding operation can be TIG,MIG, submerged arc or otherwise. Any process can be performed by powersource PS4 or other power sources using the present invention. Theelectrode can be non-consumable or consumable, such as metal cored, fluxcored or solid wire. A shielding gas may or may not be used according tothe electrode being employed. A modification of power source PS4 toperform only DC welding is illustrated as power source PS5 in FIG. 10.In this power source, welding operation 120 performs only a DC weldingoperation so that feedback loop 122 is directed to controller 170 havingan output 172. Regulated converter 100 a is preferably a chopper toproduce a DC voltage across lines 102 a, 104 a. Controller 170 iscontrolled by waveform generator 152, as shown in FIG. 9A. The polarityon lines 102 a, 104 a is either electrode negative or electrode positiveaccording to the demand of the DC welding process performed at weldingoperation 120. Regulated converter 100 a is more simplified than thewelding output of power supply PS4 shown in FIG. 9. FIGS. 9 and 10,together with the control network or circuit 150 shown in FIG. 9A,illustrates the versatility of the novel three stage power source.

It is necessary to provide a voltage for operating the controllers forboth the regulated and unregulated switching networks used in these twotypes of power sources. FIG. 11 illustrates the architecture and schemeemployed to obtain control voltages to operate the various controllersof a three stage power source, such as power source PS6. The use of anoutput of a preregulator to provide the control voltage for theswitching controller of the preregulator and switching controller of thesecond stage of a two stage power source is well known and is disclosedin Moriguchi U.S. Pat. No. 5,926,381, incorporated by reference herein.An output chopper for performing a welding operation routinely obtainsthe controller control voltage from the input DC voltage to the chopper.These two well known technologies are incorporated in power source PS6.The three stage power source can be operated with controllers havingpower supplies derived from various locations in the power source. Beingmore specific, power source PS6 has a power supply 180 with an output182 and inputs 184, 186 from the first DC bus on leads 14 a, 14 b(DC#1). Power supply 180 includes a buck converter or flyback converter,not shown, to reduce the high voltage at the output of preregulator 40of FIG. 2 to a low voltage on line 182. This control voltage may bebetween 5 and 20 volts. Voltage on line 182 is directed to controller190 having an output lead 192 for performing the operation ofpreregulator 40 in accordance with standard technology. The preregulatorhas regulation feedback lines 42, 44 shown in FIGS. 2 and 3, but omittedin FIG. 11. Unregulated inverter A does not require a controller tomodulate the duty cycle or the fixed relationship between the input andoutput voltages. However, it does require a controller 194 that receivescontroller operating voltage in line 196 from power supply 180. Thisarrangement is similar to the concept disclosed in Moriguchi U.S. Pat.No. 5,926,381, except second stage controller 194 is not a regulatingcontroller as used in the two stage power source of the prior art. As analternative, power supply PS#3 is driven by one phase of input 12 togive an optional power supply voltage shown as dashed line 176.Regulated output converter 30 of stage III has a power supply 200labeled PS#2 with a controller voltage on line 202 determined by thevoltage on DC bus 20 (DC#2) illustrated as including leads 20 a, 20 b.Again, power supply 200 includes a buck converter or flyback converterto convert the DC bus at the output of unregulated converter A to alower voltage for use by controller 210 having an output 212. The signalon line 212 regulates the output of welding converter 30 in accordancewith the feedback signal on line C, as discussed with respect to powersources PS1, PS2 in FIGS. 1 and 2, respectively. DC bus 14 (DC#1) and DCbus 20 (DC#2) provides input to power supplies 180, 200 which are DC toDC converters to produce low level DC control voltage for controllers190, 194 and 210. As an alternative shown by dashed line 220, powersupply 180 labeled PS#2 can provide control voltage for controller 210.FIG. 11 has been disclosed to illustrate the versatility of using athree stage power source with controllers that can receive reducedsupply voltages from various fixed DC voltage levels indicated to bePS#1 and PS#2. Other arrangements could be employed for providing thecontroller voltage, such as a rectified connection to one phase of ACinput voltage 12 by a transformer in a manner illustrated as PS#3.

Power source PS7 in FIG. 12 is similar to power source PS6 withcomponents having the same identification numbers. The output stage IIIis a chopper 230 for directing a DC current between electrode E andworkpiece W. Current shunt S provides the feedback signal C tocontroller 210. High switching speed inverter 240 of stage II hascharacteristics so far described with the isolation provided bytransformer 250 having primary winding 252 and secondary winding 254.The primary side of DC to DC converter 240 is the switching networkdirecting an alternating current to primary winding 252. The rectifiedoutput from secondary 254 is the secondary section or side of converter240. Converter 240 employs a high switching speed inverter that has aduty cycle or phase shift set by controller 194. The switching frequencyis about 100 kHz in the practical version of this power source. The dutycycle remains the same during the welding operation by chopper 230;however, the duty cycle or phase shift of the inverter may be adjustedas indicated by “ADJ” circuit 260 having an output 262 for adjustingcontroller 194. The duty cycle is normally close to 100% so that theswitch pairs are conductive together their maximum times at the primaryside of inverter 240. However, to change the fixed relationship betweenthe first DC bus 14 and the second DC bus 20, circuit 260 can be used toadjust the duty cycle or phase shift. Thus, the unregulated, isolationinverter 240 is changed to have a different, but fixed duty cycle.However, the duty cycle normally is quite close to 100% so the switchpairs are operated essentially in unison. The duty cycle probably variesbetween 80-100% in normal applications of the three stage power source.In the preferred implementation of the novel power source, boostconverter 62 shown in FIG. 4 is used for a power factor correcting inputstage I. This boost converter is operated in accordance with controller190 having a control voltage 182 as previously described. In accordancewith a slight modification, supply 270 has a transformer connected bylines 274 across one phase of a single phase or three phase AC input 12.A rectifier and filter in power supply 270 produces a low controlvoltage in optimal dashed line 276 for use instead of the controlvoltage in line 182 if desired. These two alternatives do not affect theoperating characteristics of power source PS7. Other such modificationsof a three stage power source for electric arc welding can be obtainedfrom the previous description and well known technology in the weldingfield.

Input stage I normally includes a rectifier and a power factorcorrecting DC to DC converter as disclosed in FIGS. 4-8. These inputstages can be used for both three phase and single phase AC signals ofvarious magnitudes, represented as input 12. Certain aspects of an inputstage for three phase AC input power are disclosed with respect to thecircuits in FIGS. 13-16. Each of these circuits has a three phase inputand a DC bus output (DC#1) that is obtained with a low harmonicdistortion factor and a high power factor for the input stage. Thedisclosure in FIGS. 1-12 are generally applicable to the novel threestage power source; however, the particular stage I used is relevant toboth a two stage power source of the prior art or the novel three stagepower source. In FIG. 13, the input circuit 300 of stage I includes athree phase rectifier 302 with output leads 302 a, 302 b. Boost switch310 is in series with an inductor 312, diode 314 and a parallelcapacitor 316. An appropriate circuit 320 which is a standard powerfactor correcting chip has an input 322 to determine the input voltage,a regulation feedback line 322 a and an output 324 for operating theboost switch to cause the current in input 12 to be generally in phasewith the input voltage. This chip is a standard three phase power factorcorrecting boost converter chip that can be used in the presentinvention and is also used for a normal two stage power source. In alike manner, input circuit 330 shown in FIG. 14 has a three phaserectifier 302 with output leads 302 a, 302 b as previously described. Aboost circuit including inductor 350, diodes 352, 354 and capacitors356, 358 are used in conjunction with switches 340, 342 to providecoordination of the current at the output of circuit 330 and inputvoltage 12. To accomplish this objective, a control chip 360 providesgating pulses in lines 362, 364 in accordance with the sensed voltage ininput 366 and feedback regulation signals in lines 367, 368. This isstandard technology to provide three phase power factor correction ofthe type that forms the input of a two stage power source or the novelthree stage power source. It has been found that the active three phasecircuits 300, 330 when operated on a three phase input provide an inputpower factor of about 0.95. The power factor of a stage I when having asingle phase AC input can be corrected upwardly to about 0.99. Since athree phase power source can generally be corrected only to a lowerlevel, it has been found that a passive circuit for the input stage I ofa two stage or three stage power source is somewhat commensurate withthe ability of an active power factor correcting circuit. A standardpassive circuit 400 is shown in FIG. 15, wherein each of the threephases is rectified by three phase rectifier 302 which directs DCcurrent through output leads 302 a, 302 b to a filter circuit includinginductor 412 and capacitor 414. It has been found that a passive circuitsuch as shown in FIG. 15 can correct the power factor of the three phaseinput to a level generally in the range of about 0.95. This is somewhatthe same as the ability of an active circuit for a three phase inputcircuit. A buck+boost input circuit 420 is shown in FIG. 16. Rectifiedcurrent on lines 302 a, 302 b is first bucked by switch 422 usingstandard power factor correcting chip 430 having a line #32 having avoltage waveform signal from input 12, that also steers chip 434 tooperate boost switch 440. Switches 422, 440 are operated in unison tocontrol the input power factor using a circuit containing inductor 450,diode 452 and capacitor 454. Circuits 300, 330, 400 and 420 are standardthree phase passive power factor correcting circuits using standardtechnology and available switches controlled by the input voltagewaveform and the current of DC#1. FIGS. 13-16 are illustrative ofcertain modifications that can be made to the first stage of the threestage power source. Of course, there is other technology for improvingthe power factor and reducing the harmonic distortion of both DC and ACsignals of the type used to drive power sources of electric arc welders.

Unregulated inverter A of stage II can use various inverter circuits.The preferred circuit is illustrated in FIG. 17 wherein the inverter isdivided between a primary section or side defined by the input toprimary winding 252 of isolating transformer 250 and a secondary sectionor side defined by output of secondary winding 254. Referring first tothe primary section or side of inverter A, full bridge circuit 500 isemployed wherein paired switches SW1-SW3 and SW2-SW4 are acrosscapacitor 548 are connected by leads 502, 504. The switches areenergized in alternate sequence by gating pulses on lines 510, 512, 514,and 516, respectively. Controller 194 outputs gating pulses in lines510-516 and an adjusted duty cycle determined by the logic on line 262from circuit 260 as previously discussed. The duty cycle is controlledby changing the phase shift of lines 510 and 512 ad lines 514 and 516.Circuit 260 adjusts the duty cycle or phase shift of the pairedswitches. This adjustment is fixed during the operation of inverter A.In practice, circuit 500 has about 100% duty cycle or phase shift, whereeach pair of switches has maximum periods of overlapping conduction.Controller 194 has a control voltage from an appropriate supplyindicated by line 196, as also previously described. In operation ofcircuit 500, an alternating current is directed through primary winding252. This current has an ultra high frequency normally at least about100 kHz so the components can be reduced in size, weight and cost. Thehigh switching frequency is not dictated by the welding operation, butis selected for efficiency of unregulated stage A of the three stagepower source. A block capacitor is in series with the primary winding toprevent saturation with unregulated gate drive signals. The secondarysection or side of inverter A is a rectifier 520 having synchronousrectifier devices 522, 524. Synchronous rectifier devices are well knownin the general electrical engineering art and are discussed in BoylanU.S. Pat. No. 6,618,274 incorporated by reference herein. These devicesare gated by signals on lines 526, 528 created at the opposite ends ofsecondary winding 254 in accordance with standard technology. Leads 530,532, and 534 form the output leads of rectifier 520 to create a DCvoltage (DC#2) across leads 20 a, 20 b. The current is smooth by a choke544 and is across capacitor 546, in accordance with standard weldingtechnology. Inverter A is unregulated which means that it is notadjusted by a real time feedback signal from the welding operation. Itmerely converts DC bus 12 (DC#1) to DC bus 20 (DC#2). This conversionallows a substantial reduction in the voltage directed to the regulatedthird stage of the power source using inverter A. The reduction involtage is primarily determined by the turns ratio of transformer 250,which ratio, in the preferred embodiment, is about 4:1. For DC #1, thevoltage is around 400 volts. Thus, the fixed voltage on output bus 20 isabout ¼ the fixed voltage on output bus 12 of the first stage. Severaladvantages of an unregulated stage are contained in an article entitledThe incredible Shrinking (Unregulated) Power Supply by Dr. Ray Ridleyincorporated by reference herein as background information. A basicadvantage is the ability to increase the frequency to above 100 kHz toreduce the size and cost of the inverter stage.

Various circuits can be used for the unregulated inverter A constitutingnovel stage II of the invention. The particular type of inverter is notcontrolling. Several inverters have been used. Some are illustrated inFIGS. 18-21. In FIG. 18, inverter A is shown as using a full bridgecircuit 600 on the primary side of transformer 250. A switch and diodeparallel circuit 602, 604, 606 and 608 are operated in accordance withthe standard phase shift full bridge technology, as explained withrespect to the inverter A version shown in FIG. 17. A modification ofthe internal workings for inverter A is illustrated in FIG. 19 utilizinga cascaded bridge with series mounted switch circuits 610, 612 and 614,616. These switch circuits are operated similar to a half bridge andinclude input capacitors 548 a, 548 b providing energy for the switchingcircuits which in parallel is capacitor 620 and is in series with diode622, 624. The two switch circuits are in series so there is a reducedvoltage across individual switches when a phase shift control techniquesimilar to the technique for the full bridge inverter of FIG. 17 isused. This type of inverter switching network is illustrated inCanales-Abarca U.S. Pat. No. 6,349,044 incorporated by reference hereinshowing an inverter using a cascaded bridge, sometimes referred to as athree level inverter. A double forward inverter is shown in FIG. 20wherein switches 630, 632 provide a pulse in section 252 a of theprimary winding for transformer 250 a. In a like manner, switches 634,636 are operated in unison to provide an opposite polarity pulse inprimary section 252 b. The alternating pulse produces an AC at theprimary winding of transformer 250 a to produce an isolated DC output insecondary winding 254. A standard half bridge circuit is shown as thearchitecture of inverter A in FIG. 21. This half bridge includesswitches 640, 642 alternately switched to produce an AC in primarywinding 252 of transformer 250. These and other switching circuits canbe used to provide an AC signal in the primary winding of transformer250 so that the secondary isolated AC signal is rectified and outputtedon leads 20 a, 20 b as DC#2. The mere description of certainrepresentative standard switching networks is not considered to beexhaustive, but just illustrative. Control of the welding current is notperformed in the second stage. In this stage, a DC bus having a highvoltage is converted to a fixed DC bus (DC#2) having a low voltage forthe purposes of driving a third stage, which third stage is a regulatedstage to provide a current suitable for electric arc welding. Electricarc welding incorporates and is intended to include other weldingrelated applications, such as the concept of plasma cutting. The variouscircuits used in the three stages can be combined to construct variousarchitectures for the basic topography which is a three stage powersource.

PREFERRED EMBODIMENT FIGS. 22-24

This description relates to FIGS. 22-27 and uses the numbers of thosefigures to indicate like components, while using some relevant numbersfrom FIGS. 1-17. The 600 numbers in FIGS. 18-21 are not used for thesame components in FIGS. 22-27. In FIG. 22 the first two stages of theimproved three stage power source includes unregulated converter A asbest shown in FIG. 17 wherein the input DC signal across lines 14 a, 14b is provided by a novel first input stage shown as boost converter 600(FIG. 22) having power switch 602 (FIG. 22) switched by a gate signal inline 604 (FIG. 22). Switch 602 (FIG. 22) is turned on after auxiliaryswitch 628 is turned on. The timing of gating signals in lines 192 and192 a is by power factor correcting controller 194. A high frequencysignal in line 192 causes a high frequency switching signal in gate 604(FIG. 22) of main power switch 602 (FIG. 22) with anti-parallel diode602 a, in accordance with standard boost technology. The timing of thesignal on gate 604 (FIG. 22) is controlled in accordance with previousdiscussions to obtain power factor correction for the power supplycreating the rectified signal on input leads 12 a, 12 b. The DC signalat leads 12 a, 12 b is converted by switch 602 and output rectifierdiode 610 (FIG. 22) into a DC bus at leads 14 a, 14 b. The inventioninvolves the use of an active soft switching circuit 620 (FIG. 22)having a network including a first branch with inductor 622 (FIG. 22)and a second branch with parasitic capacitor 624 (FIG. 22). The networkis actuated by series connected auxiliary switch 628. Some discussionsidentify this two branch network as a tank circuit or resonant circuit.This is technically justified but not necessary to the soft switchingfunction. Capacitor 624 (FIG. 22) and inductor 622 (FIG. 22) form afilter circuit for soft switching 628 (FIG. 22) wherein capacitor 640causes a soft voltage turn on of boost diode 610 (FIG. 22) by way ofdiode D2. This boost diode is sometimes referred to as an output orrectifier diode. Circuit 620 (FIG. 22) is an active soft switchingcircuit controlling the voltage and current across power switch 602(FIG. 22) events and also across output diode 610. Thus, power switch602 (FIG. 22) and boost or output diode 610 (FIG. 22) in boost converter600 (FIG. 22) are commutated with soft switching. This feature makes theswitching technique particularly attractive for high voltage conversionapplications where the boost diode suffers from severe reverse recoveryproblems. For instance, in a power factor correcting boost circuit, boththe power switch and the rectifier diode are subject to high voltages.With the conventional pulse width modulator technique employed, due tothe reverse recovery of minority-rectifier diode 610 (FIG. 22), highswitching loss, high EMI noise, and device failure problems become morepronounced. Therefore implementation of the soft switching for bothpower switch 602 (FIG. 22) and diode 610 (FIG. 22) is beneficial. Thevoltage and current waveforms of the switches in the converter areessentially square wave except during the turn on and turn off switchingintervals when the zero voltage switching transition takes place. Boththe power switch and the boost diode are subject to a minimum voltageand current stress. Auxiliary switch 628 can be very small compared tothe main switch, as it only handles small amounts of resonant-transitionenergy. Since soft switching is achieved without increasing switchingvoltage and current stresses, there is no substantial increase in theconduction loss when using active circuit 620 (FIG. 22). Basically,circuit 620 (FIG. 22) is selected to provide soft switching in bothcurrent and voltage at transitions of power switch 602 (FIG. 22) and,optionally, at transitions of output diode 610 (FIG. 22). Thus, a twostage converter is used to convert the DC signal on lines 12 a, 12 b toa DC signal in lines 20 a, 20 b. The efficiency of this two stage deviceis drastically increased by having a soft switching circuit on boostconverter 600 (FIG. 22) and using the inherent soft switching ofunregulated inverter A. Consequently, the two stage DC to DC convertershown in FIG. 22 is a substantial improvement for the input side of athree stage welding power source. In operation, a high frequencyswitching signal in line 192, which signal exceeds 18 kHz, firstenergizes auxiliary switch 628 by the gating signal in line 192 a toactivate the resonant tank circuit formed by inductor 622 (FIG. 22) andcapacitor 624 (FIG. 22). After switch 628 has been turned on, mainswitch 602 (FIG. 22) is turned on. This causes soft switching both incurrent and voltage. At the same time, the passive portion of circuit620 (FIG. 22) controls the voltage and current across output rectifierdiode 610 (FIG. 22). The positive polarity side of auxiliary switch 628is clamped to capacitor 640 (FIG. 22) by diode D1. This clamps the softswitching circuit to the positive output, the circuit includinginductance and capacitance branches does not float during operation. Thecircuit shown in FIG. 22 is discussed in the 1991 IEEE article entitledHigh Efficiency Telecom Rectifier Using a Novel Soft-SwitchingBoost-Based Input Current Shaper. This article is being incorporated byreference herein. A similar soft switching circuit for the power switch602 (FIG. 22) is described in a 2004 IEEE article entitled A NewZVT-ZCT-PWM DC-DC Converter. This similar type active soft switchingcircuit used for power switch 602 (FIG. 22) is shown in FIG. 23 whereinthe numbers for the same components as shown in FIG. 22 are the same.The 600 numbers are not those in FIGS. 18-21. Active soft switchingcircuit 700 has resonant inductors 704, 706 divided into segments andcoupled by common core 705. Current controlling diodes 704 a, 706 a,respectively. These diodes are in series with the inductors which are,in turn, parallel with parasitic capacitance 708. Auxiliary switch 710has an anti-parallel diode 712 so that switch 710 operates in accordancewith the previously discussed auxiliary switch 628 of FIG. 22. Softswitching circuit 700 includes voltage control capacitor 720 forcontrolling the voltage across output rectifier diode 610 (FIG. 22). Toclamp the positive side of auxiliary switch 710 to output lead 14 a,there is provided a single diode 730. This diode operates as diode D1,D2 in FIG. 22. Soft switching circuit 700 provides soft switching, bothvoltage and current across power switch 602 (FIG. 22) and control thevoltage and current during the switching of rectifier diode 610 (FIG.22). Thus, circuit 700 essentially operates in the same fashion aspreviously discussed soft switching circuit 600 (FIG. 22). The presentinvention involves an active soft switching circuit for the power switch602 (FIG. 22) and optionally for the rectifier diode 610 (FIG. 22). Thetopography for the soft switching circuits may vary, with two of thepreferred soft switching circuits 600 (FIG. 22), 700, illustrated inFIGS. 22, 23, respectively. The switches SW1, SW2, SW3 and SW4 are solidstate switches with an anti-parallel diode, such as diode 602 a.Furthermore, a capacitor 506 a prevents saturation of transformer core250 a.

By providing an active soft switching circuit for the boost input stageof the three stage power source, the operation of the input stagecombines with the inherent soft switching characteristics of the secondunregulated inverter stage to provide a two stage input that improvesthe efficiency of the novel three stage power source, as described inFIGS. 1-21. It has been found that circuit 700 pushes the voltage downcloser to zero during high speed switching of switch 602 (FIG. 22).Circuit 600 (FIG. 22) lowers the voltage, but the voltage duringswitching using circuit 600 (FIG. 22) is not exactly zero. Indeed, itmay be as high as about 50 volts. Consequently, soft switching circuit600 (FIG. 22) is preferred because of its lower cost and soft switchingcircuit 700 is an alternative because of its ability to push the actualvoltage down near zero during the switching of switch 602 (FIG. 22).These distinctions are the reasons for illustrating two separate activesoft switching circuits for use on the input stage of the novel threestage power source as described above.

The three phase power source as described in FIG. 12 is illustratedagain in FIG. 24, using like numbers, wherein chopper 230 is shown ashaving power switch 750 controlled by high frequency gating signal online 212 from controller 210. A feedback signal on line 762 from currentsensing device 760 is generated by readings of shunt S. In a likemanner, a voltage feedback signal is directed to controller 210 by line772 from a voltage sensing device 770. These two feedback signalscontrol the operation of a pulse width modulator in controller 210 foroperating power switch 750 of chopper 230. Input capacitor 780 controlsthe voltage across input leads 20 a, 20 b in accordance with standardpractice. An optional aspect of the invention is providing a passivesoft switching circuit 800 for chopper 230, which passive soft switchingof the chopper is combined with the active soft switching of the inputstage and the inherent soft switching of the second stage to increasethe efficiency of the three stage power source shown in FIG. 12 anddescribed in FIGS. 1-21. Soft switching circuit 800 is a commonly usedsoft switching circuit. The circuit includes inductor 802 forcontrolling current across the power switch and diode D4. Capacitor 806controls the voltage across the power switch during the switchingoperations. Capacitors 804 and 806 are connected by diodes D1, D2, D3and D4. These two capacitors control the voltage across diode D4. Thus,power switch 750 and diode D4 are soft switched in both current andvoltage during switching operations. This circuit is shown in theUniversity of California article entitled Properties and Synthesis ofPassive, Lossless Soft-Switching PWM Converters. This May 1997 articleis incorporated by reference herein to explain the operation of thecommonly used passive soft switching circuit 800. In essence, chopper230 has a power switch with a soft switching circuit to control both thecurrent and voltage during turn-on and turn-off transitions. In otherwords, output chopper 230 is provided with a soft switching circuit,which soft switching circuit controls both voltage and current at theappropriate time during the switching operations.

The three stage power source described in FIGS. 1-21 is provided with aninput stage having an active soft switching circuit which combines withthe inherent soft switching of unregulated inverter A of the secondstage to increase the overall efficiency by reducing the switchinglosses and conduction losses at the input side of the power source. Asan option, the chopper output stage is provided with a passive softswitching circuit to provide an inexpensive final stage. The chopper maybe a separate, replaceable module without the need for a circuitmodification to control an auxiliary switch as required in an activesoft switching circuit. The input portion of the three stage powersource includes an active power factor correcting stage combined with anunregulated phase shift pulse width modulating stage. This novelcombination of the first two stages is highly efficient and inexpensiveas a topography for electric arc welders.

FIGS. 25 and 26, the show first stage 600 is of FIG. 22 as a boost typeDC to DC converter including an inductor 644 coupled between the inputlead 12 a and a main internal node 603, a main switching device 602(FIG. 22) with a body diode 602 a coupled between the internal node 603and the lower converter input lead 12 b. A main rectifier diode 610(FIG. 22) is coupled with its anode at node 603 and its cathode atoutput lead 14 a. Optional output filter capacitor 548 is connectedacross the output leads 14 a, 14 b. As in normal boost converteroperation, main switch 602 (FIG. 22) is activated by a pulse widthmodulated (PWM) control signal at a control gate thereof to switchbetween a conducting (ON) state in which the internal node 603 isbrought essentially to the voltage at the lower lead 12 b (chargephase), and a non conducting (OFF) state (discharge phase). Prior toeach charging phase, assuming that the main switching device 602 (FIG.22) has been in the non conducting state (OFF) for a relatively longtime, the voltage across output capacitor 548 is equal to the inputvoltage plus the voltage of inductor 644. Closure of the main switch 602(FIG. 22) brings node 603 essentially to the voltage of lower lead 12 b,whereby the input voltage is impressed across inductor 644 (terminal 12a is positive with respect to node 603) and main diode 610 FIG. 22)prevents filter capacitor 548 from discharging through main switch 602(FIG. 22). The voltage across inductor 644 causes the currenttherethrough to rise over time, with the corresponding energy beingstored in inductor 644. Thereafter, main switch 602 (FIG. 22) isdeactivated (OFF) to begin a discharge phase. Placing switch 602 (FIG.22) in the non conducting state causes the main inductor voltage tochange such that the voltage at node 603 rises to maintain the currentthrough inductor 644 at a constant value, wherein for the inductorcurrent to continue flowing, the voltage at node 603 must rise enough toforward bias diode 610, as shown in FIG. 22 (e.g., approximately theoutput voltage across capacitor 548 plus a diode drop), wherein theinductor voltage changes polarity in the discharge phase. For largeoutput capacitance 548, the output voltage between leads 14 a and 14 bremains generally constant during the discharge phase, wherein thecharging and discharging (switching of main switch 602 (FIG. 22) on andoff) is repeated with appropriate feedback to regulate the pulse widthmodulation of the switch control signal, such that output voltage acrossthe capacitor 548 can be maintained at a desired DC value.

In general, it is desirable to maximize the efficiency of each stage inthe power source, wherein the on state resistance of main switch 602(FIG. 22), the diode forward voltage drop, and the reverse recovery timeratings for main diode 610 (FIG. 22) are ideally minimized to combatconduction losses. Another consideration is minimization of switchinglosses and noise generation in converter stage 600 (FIG. 22), wherein itis desirable to control the conditions under which the state transitionsof switch 602 (FIG. 22) and diode 610 (FIG. 22) occur. In particular,soft switching circuits may be advantageously employed in boostconverter 600 (FIG. 22) to provide zero voltage switch turn on and turnoff, as well as zero voltage or zero current turn off of diode 610 (FIG.22). Absent counter measures, the switching of main switch 602 (FIG. 22)causes undesirable power loss and stress to switch 602 (FIG. 22) and/orto main diode 610 (FIG. 22). Accordingly, soft switching or snubbercircuitry is employed in boost converter stage 600 (FIG. 22) to providelow current and or low voltage switching of these components. In thisregard, soft switching circuitry may be used to minimize the rate ofvoltage rise across switch 602 as shown in FIG. 22 (e.g., dv/dt at node603) when switch 602 (FIG. 22) is turned off, and to minimize thevoltage across switch 602 when switch 602 (FIG. 22) is turned on, aswell as to minimize one or both of the voltage or current of diode 610(FIG. 22) during reversal thereof, in order to minimize switching lossesand noise emission.

The soft start switching circuit shown in Hua U.S. Pat. No. 5,418,704can be used in boost stage 600 of the three stage power source asschematically shown in FIG. 24. This patent is incorporated by referenceand is different from the first embodiment circuit shown in FIG. 25 andthe preferred embodiment circuit shown in FIG. 26. The soft switchingcircuit described in Hua U.S. Pat. No. 5,418,704 employs an auxiliaryswitch with a resonant inductor and capacitor to provide zero-voltageswitching of the boost converter main switch and the output diode. Thisis a publication referring to the two branch network of the invention asa resonant circuit. In Hua, the auxiliary switch and the resonantinductor are connected in series across the main converter switch. Theauxiliary switch is switched on immediately prior to turning on the mainswitch so the resonant inductor is diode coupled to the positiveconverter output lead to limit the rate of change of the main diodecurrent. Activation of the auxiliary switch of Hua also discharged theinternal node to zero volts, thereby ensuring that the main switch wasturned on at essentially zero voltage. However, Hua suffers from hardswitching conditions during main transistor turn-off. In particular, theupper main switch terminal voltage of Hua must be higher than theconverter output voltage before the resonant inductor can conduct anycurrent to the output, whereby the resonant inductor of Hua causes veryfast transistor voltage rise (hi dv/dt) during transistor turn off,leading to unacceptable switching losses.

As illustrated in FIGS. 25 and 26, exemplary boost converter stage 600(FIG. 22) includes an active soft switching circuit 601 or 601 a,respectively, for providing soft switching of main switch 602 (FIG. 22)and main diode 610 (FIG. 22). The exemplary soft switching circuit 601in FIG. 25, which is the original version of the invention, is a threeterminal network having first and second terminals coupled across mainswitch 602 (FIG. 22) and a third terminal coupled to the cathode of maindiode 610 (FIG. 22). The soft switching circuit or network includesinductor 622 (FIG. 22), auxiliary switching device 628 with diode 630(FIG. 22). First and second diodes D1 and D2, along with capacitors 624and 640 (FIG. 22) complete a three terminal snubber circuit. Main andauxiliary switching devices 602 (FIG. 22) and 628 can be any suitabledevices that selectively provide generally conductive and generally nonconductive states between first and second power terminals thereofaccording to a control signal at a control terminal thereof, including,but not limited to, bipolar transistors, metal oxide semiconductor (MOS)devices, isolated gate bipolar transistors (IGBTs) and the like.Inductor 622 (FIG. 22) is in a first branch in parallel with switch 602(FIG. 22). Inductor 622 (FIG. 22) has a first terminal coupled with maininductor 644 and a second terminal attached to a first intermediatecircuit node 607. Auxiliary switching device 628 is coupled between node607 and converter leads 12 b, 14 b. Diode 630 (FIG. 22) may be a bodydiode of auxiliary switching device 628 or may be a separate component.An anode of diode 630 (FIG. 22) is coupled to lower converter leads 12b, 14 b and its cathode is coupled to node 607 at the connection of theauxiliary switch 628 and the resonant inductor 622 (FIG. 22). Similar tothe circuit of Hua, one capacitor 624 (FIG. 22) is coupled in thecircuit 601 across main switch 602 (FIG. 22). Unlike Hua, however, softswitching circuit 601 in FIG. 25 has a second intermediate node 609 withsecond capacitor 640 (FIG. 22) coupled between nodes 603 and 609. Firstdiode D1 of soft switching circuit or network 601 has an anode coupledwith first internal node 607 and a cathode coupled with second internalnode 609. Diode D2 has an anode coupled with second internal node 609and a cathode coupled to the cathode of main diode 610 (FIG. 22) at theupper converter output terminal 14 a.

As a technical advance over Hua with its hard switching of the auxiliaryswitch, soft switching circuit 601 of FIG. 25 provides soft switchingoperation for turn on and turn off of both main switch 602 (FIG. 22) andmain diode 610 (FIG. 22) as well as auxiliary switch 628. Thisimprovement achieves better efficiency, lower component stresses, andless noise generation. Prior to turning on main switch 602 (FIG. 22),auxiliary switch 628 is switched on while the voltage at node 603 isequal to the output voltage, where the closure of the auxiliary switch628 causes the current through resonant inductor 622 (FIG. 22) to riseinitially to the main inductor current level, by which main diode 610(FIG. 22) reverses. As diode 610 (FIG. 22) recovers the voltage reversaland begins to block current from the output, the current from inductors644 and 622 (FIG. 22) discharges capacitor 624 (FIG. 22), wherein thevoltage across diode 610 (FIG. 22) remains small during the reversal tominimize the diode switching loss and noise generation. Main switch 602(FIG. 22) is then switched on when capacitor 624 (FIG. 22) is discharged(e.g., when the voltage across switch 602 (FIG. 22) is zero), andauxiliary switch 628 is turned off. The current through the resonantinductor 622 (FIG. 22) charges first resonant capacitor 640 (FIG. 22)through diode D1 and also charges any parasitic capacitance of theauxiliary switch 628, whereby the voltage at nodes 607 and 609 risetoward the level of the converter output and diode D2 begins to conduct.Any remaining energy from the inductor 622 (FIG. 22) is provided to theoutput through diodes D1 and D2. Main switch 602 (FIG. 22) is thenturned off (at a time dependent upon the current pulse width modulationbased on output level feedback) while the switch voltage is essentiallyzero. The current through main inductor 644 charges capacitor 624 (FIG.22) and discharges resonant capacitor 640 (FIG. 22) through diode D2.This action causes the voltage at node 607 to rise to the output value,after which main diode 610 (FIG. 22) again begins to conduct current tothe output.

In operation of the circuit 601 of FIG. 25, the main inductor currentflows through capacitor 640 (FIG. 22) and second diode D2 when mainswitch 602 (FIG. 22) is initially turned off, where main diode 610 (FIG.22) begins to conductor after resonant capacitor 640 (FIG. 22)discharges, wherein the voltage across first capacitor 640 (FIG. 22) isa function of its capacitance, the main current level, and the dutycycle of pulse width modulated main switch 602 (FIG. 22). In thismanner, the switching losses of main diode 610 (FIG. 22) can be reducedor minimized by ensuring zero diode voltage when the diode begins toconduct current to output capacitor 548. With main switch 602 (FIG. 22)in the on state, the voltage across first resonant capacitor 640 (FIG.22) remains generally constant because first diode D1 prevents capacitorcharging, except when auxiliary switch 628 is first turned off and thevoltage at node 607 is higher than the voltage across the capacitor 640(FIG. 22). Ideally, main switch 602 (FIG. 22) has a zero voltage turnoff condition if resonant capacitor 640 (FIG. 22) is fully dischargedduring the boost phase with switch 602 (FIG. 22) on. However, mainswitch 602 (FIG. 22) will experience a non zero turn off voltage ifresonant capacitor 640 (FIG. 22) is not fully discharged. In addition,capacitor 640 (FIG. 22) may only provide a current bypass path forauxiliary inductor 622 (FIG. 22) when auxiliary switch 628 is turnedoff, without providing sufficient bypass conduction path for parasiticinductances in the auxiliary circuit loop in soft switching circuit 606of FIG. 25. As a result, the transition of auxiliary switch 628 from onto off may be at a non zero voltage, whereby switching losses and noisegeneration are possible, along with possible stress to switch 628.

FIG. 26 illustrates the preferred embodiment and preferred design of thesoft switching circuit 601 a, and in accordance with the invention, hascapacitor 624 (FIG. 22) removed. A second capacitor 640 a is coupledbetween internal node 609 and lower converter leads 12 b, 14 b, wherebya net capacitance results from the series combination of capacitors 640(FIG. 22) and 640 a, with this series combination being a branchparallel across main switch 602 (FIG. 22). Lower (second) capacitor 640a is in parallel across auxiliary switch 628 via diode D1. In oneparticular implementation, lower capacitor 640 a is substantiallysmaller than the upper capacitor 640 (FIG. 22). Thus, unlike the softswitching network of FIG. 25, circuit 601 a of FIG. 26 providescapacitor 624 (FIG. 22) between second internal node 609 and lowerconverter leads 12 b, 14 b as two capacitors 640 (FIG. 22), 640 a. Thisgeometry aids in providing soft switching for auxiliary switch 628(e.g., reduces dv/dt across switch 628).

Referring now to FIG. 27, graph 900 illustrates various exemplarywaveforms associated with main and auxiliary switches 602 (FIG. 22) and628, respectively, in boost converter stage 600. The exemplary activesoft switching circuit 601 a of FIG. 26 is also shown. The graph 900shows voltage waveform 810 corresponding to an auxiliary switch controlvoltage signal (e.g., gate signal VGS, base signal VBE, etc., dependingon switch type), voltage waveform 820 representing the voltage acrossauxiliary switch 628 (e.g., the voltage between internal node 607 andlower converter leads 12 b, 14 b), and current waveform 830 illustratingthe current switched through auxiliary switch 628. In addition, graph800 also provides voltage waveform 840 showing a control voltage signalfor main switch 602 (FIG. 22) as well as a voltage waveform 850representing the voltage across main switch 602 (e.g., the voltagebetween node 603 and lower converter leads 12 b, 14 b.

Various discreet times are illustrated in a typical switching cycle ofconverter stage 600 (FIG. 22) in the graph 900, including time 870, atwhich time main switch 602 (FIG. 22) is turned off (e.g., falling edgeof voltage waveform 840), time 872 when auxiliary switch 628 is turnedon (rising edge on control signal 810), and time 874 when auxiliaryswitch 628 is turned off and main switch 602 (FIG. 22) is turned on(falling edge on waveform 810 and rising edge on waveform 840). Whileillustrated as being switched simultaneously at time 874, auxiliaryswitch 628 may alternatively be turned off prior to, concurrently with,or after the time when main switch 602 (FIG. 22) is turned on, whereinall such variant implementations are deemed as falling within the scopeof the invention and the appended claims. In the illustratedimplementation of the circuit shown in FIG. 26, main switch 602 (FIG.22) is turned off at time 870, after which the voltages across main andauxiliary switches 602 (FIG. 22) and 628 (e.g., voltages at nodes 603and 607) rise as indicated in graph 900 at portion 852 and portion 822,respectively. It is noted that the voltage curve 850 is zero across mainswitch 602 (FIG. 22) during main switch turn on at time 870, whereby anycorresponding switching losses and/or noise emission are mitigated. Asshown in FIG. 27, switch voltage curves 820 and 850 remain essentiallyconstant at portion 824 and portion 854 with a value generally equal tothe value of the voltage across output filter capacitor 548 (VOUT) untiltime 872 when auxiliary switch 628 is turned on (with main switch 602(FIG. 22) remaining off), whereby the auxiliary switch voltage drops tozero at point 826. It is noted that auxiliary switch current curve 830is essentially zero at time 872, whereby auxiliary switch 628 suffers nosignificant turn on switching loss. Thereafter, at time 874, main switch602 (FIG. 22) is again turned on. It is noted that between time 872 andtime 874, main switch voltage curve 850 drops generally at portion 856to zero prior to switch 602 (FIG. 22) being turned on, whereby a zerovoltage turn on condition is provided to minimize switch loss and noisegeneration by main switch 602 (FIG. 22). Moreover, unlike the circuit ornetwork 601 in FIG. 25 above, auxiliary current curve 830 initiallyrises at portion 832 after the auxiliary switch turn on time 872, but isthen reduced to zero at portion 834 prior to the auxiliary switch turnoff time 874, whereby the auxiliary switch turn off is a soft switchingevent with minimized (e.g., zero) switching loss and noise emission.Main switch 602 (FIG. 22) is then turned on at 874 at essentially zerovolts, and the auxiliary switch voltage 820 rises at portion 828 until atime 876 at which the current through resonant inductor 622 (FIG. 22)falls to zero. Thereafter, the cycle continues until the next time 870,at which main switch 602 (FIG. 22) is again turned off, wherein theamount of time that main switch 602 (FIG. 22) remains on in a givenswitching cycle may be determined by output regulation conditionsthrough pulse width modulation or other suitable techniques. Circuit 601a of FIG. 26 provides soft switching of auxiliary switch 628 while theauxiliary switch 628 in circuit 601 has a hard turn off. This is adistinct improvement obtained by the preferred circuit 601 a of FIG. 26.

The soft switching system or network 601, 601 a of FIGS. 25 and 26,respectively, includes two parallel branches parallel to main powerswitch 602 (FIG. 22). A first branch includes the inductance of inductor622 (FIG. 22) controlling the current to auxiliary switch 628, switch602 (FIG. 22) and diode 610 (FIG. 22), while the second branch has acapacitance controlling voltage across switch 602. In FIG. 26, thisparallel branch is divided into two capacitors, one of which controlsthe voltage across auxiliary switch 628.

The capacitance of capacitors 640 (FIG. 22), 640 a of FIG. 26 generallyequals the capacitance of capacitor 624 of FIG. 25. Capacitor 640 (FIG.22) soft switches switch 628 as it is turned off. As switch 628 isturned off capacitor 640 a is at zero voltage. It charges slowly toprovide soft turnoff. When switch 628 is turned on, current in theswitch increases slowly through inductor 622 (FIG. 22) and diode 610(FIG. 22) is turned off slowly by slow current rise in the inductor.Thus, network 601 a soft switches switch 628 during on and off cyclesand controls current through boost or output diode 610 (FIG. 22). Thisis an improvement over network 601 of FIG. 25.

The various switching circuits and power source topologies disclosed canbe combined in several ways to accomplish the objectives and advantagesof the claimed invention.

Having thus defined the invention, the following is claimed:
 1. A threestage power source for an electric arc welding process, said powersource comprising an input stage having an AC input and a first fixed DCoutput signal; a second stage in the form of an unregulated DC to DCconverter having an input connected to said first fixed DC outputsignal, a network of switches switched at a high frequency with a givenduty cycle to convert said input into a first internal AC signal, anisolation transformer with a primary winding driven by said firstinternal high frequency AC signal and a secondary winding for creating asecond internal high frequency AC signal and a rectifier to convert saidsecond internal AC signal into a second fixed DC output signal of saidsecond stage; with a magnitude related to said duty cycle of saidswitches; and a third stage to convert said second fixed DC outputsignal to a welding output for welding wherein said input stage has aregulated DC to DC converter including a boost converter having a maininductance, a main rectifier, and a power switch having an active softswitching circuit, said active soft switching circuit of said inputstage comprising: an auxiliary switching device and a tank circuitclosed by said auxiliary switching device, said tank circuit comprisinga resonant inductance coupled in series with said auxiliary switchingdevice, and a resonant capacitance coupled in parallel with said powerswitch, said resonant capacitance having a first resonant capacitorportion coupled in parallel with said resonant inductance and a secondresonant capacitor portion coupled in parallel with said auxiliaryswitching device.
 2. A three stage power source as defined in claim 1wherein said regulated DC to DC converter is a power factor correctingconverter.
 3. A three stage power source as defined in claim 1 whereinsaid regulated DC to DC converter is said boost converter.
 4. A threestage power source as defined in claim 1 wherein said tank circuit isactivated by said auxiliary switch.
 5. A three stage power source asdefined in claim 1 wherein said auxiliary switch is operated in unisonwith said power switch.
 6. A three stage power source as defined inclaim 1 wherein said third stage is a chopper with a power switch havinga passive soft switching circuit.
 7. A three stage power source asdefined in claim 1 wherein said given duty cycle is greater than 80% tohold conduction losses of said second stage at a low level.
 8. A threestage power source as defined in claim 1, wherein said given duty cycleis adjustable.
 9. A three stage power source as defined in claim 2wherein said regulated DC to DC power factor correcting converterincludes said boost converter.
 10. A three stage power source as definedin claim 2 wherein said tank circuit is activated by said auxiliaryswitch.
 11. A three stage power source as defined in claim 2 whereinsaid auxiliary switch is operated in unison with said power switch. 12.A three stage power source as defined in claim 2 wherein said thirdstage is a chopper with a power switch having a passive soft switchingcircuit.
 13. A three stage power source as defined in claim 2 whereinsaid given duty cycle is greater than 80% to hold conduction losses ofsaid second stage at a low level.
 14. A three stage power source asdefined in claim 3 wherein said auxiliary switch operated in u nisonwith said power switch.
 15. A three stage power source as defined inclaim 3 wherein said tank circuit is activated by said auxiliary switch.16. A three stage power source as defined in claim 3 wherein said thirdstage is a chopper with a power switch having a passive soft switchingcircuit.
 17. A three stage power source as defined in claim 3 whereinsaid given duty cycle is greater than 80% to hold conduction losses ofsaid second stage at a low level.
 18. A three stage power source asdefined in claim 5 wherein said main rectifier is a boost diode softswitched by said soft switching circuit.
 19. A three stage power sourceas defined in claim 5 wherein said regulated DC to DC converter has apositive and a negative output lead with said resonant capacitancejoining said leads and a diode clamping the positive end of saidauxiliary switch to said positive output lead.
 20. A three stage powersource as defined in claim 5 wherein said third stage is a chopper witha power switch having a passive soft switching circuit.
 21. A threestage power source as defined in claim 5 wherein said given duty cycleis greater than 80% to hold conduction losses of said second stage at alow level.
 22. A three stage power source as defined in claim 9 whereinsaid tank circuit is activated by said auxiliary switch.
 23. A threestage power source as defined in claim 9 wherein said auxiliary switchis operated in unison with said power switch.
 24. A three stage powersource as defined in claim 9 wherein said third stage is a chopper witha power switch having a passive soft switching circuit.
 25. A threestage power source as defined in claim 9 wherein said given duty cycleis greater than 80% to hold conduction losses of said second stage at alow level.
 26. A three stage power source as defined in claim 11 whereinsaid main rectifier is a boost diode soft switched by said softswitching circuit.
 27. A three stage power source as defined in claim 11wherein said regulated DC to DC converter has a positive and a negativeoutput lead with said resonant capacitance joining said leads and adiode clamping the positive end of said auxiliary switch to saidpositive output lead.
 28. A three stage power source as defined in claim11 wherein said third stage is a chopper with a power switch having apassive soft switching circuit.
 29. A three stage power source asdefined in claim 11 wherein said given duty cycle is greater than 80% tohold conduction losses of said second stage at a low level.
 30. A threestage power source as defined in claim 14 wherein said main rectifier isa boost diode soft switched by said soft switching circuit.
 31. A threestage power source as defined in claim 14 wherein said tank circuitclosed is activated by said auxiliary switch.
 32. A three stage powersource as defined in claim 14 wherein said regulated DC to DC converterhas a positive and a negative output lead with said resonant capacitancejoining said leads and a diode clamping the positive end of saidauxiliary switch to said positive output lead.
 33. A three stage powersource as defined in claim 14 wherein said third stage is a chopper witha power switch having a passive soft switching circuit.
 34. A threestage power source as defined in claim 14 wherein said given duty cycleis greater than 80% to hold conduction losses of said second stage at alow level.
 35. A three stage power source as defined in claim 18 whereinsaid regulated DC to DC converter has a positive and a negative outputlead with said resonant capacitance joining said leads and a diodeclamping the positive end of said auxiliary switch to said positiveoutput lead.
 36. A three stage power source as defined in claim 18wherein said third stage is a chopper with a power switch having apassive soft switching circuit.
 37. A three stage power source asdefined in claim 18 wherein said given duty cycle is greater than 80% tohold conduction losses of said second stage at a low level.
 38. A threestage power source as defined in claim 23 wherein said main rectifier isa boost diode soft switched by said soft switching circuit.
 39. A threestage power source as defined in claim 23 wherein said regulated DC toDC converter has a positive and a negative output lead with saidresonant capacitance joining said leads and a diode clamping thepositive end of said auxiliary switch to said positive output lead. 40.A three stage power source as defined in claim 23 wherein said thirdstage is a chopper with a power switch having a passive soft switchingcircuit.
 41. A three stage power source as defined in claim 23 whereinsaid given duty cycle is greater than 80% to hold conduction losses ofsaid second stage at a low level.
 42. A three stage power source asdefined in claim 26 wherein said regulated DC to DC converter has apositive and a negative output lead with said resonant capacitancejoining said leads and a diode clamping the positive end of saidauxiliary switch to said positive output lead.
 43. A three stage powersource as defined in claim 26 wherein said third stage is a chopper witha power switch having a passive soft switching circuit.
 44. A threestage power source as defined in claim 26 wherein said given duty cycleis greater than 80% to hold conduction losses of said second stage at alow level.
 45. A three stage power source as defined in claim 30 whereinsaid tank circuit is activated by said auxiliary switch.
 46. A threestage power source as defined in claim 30 wherein said regulated DC toDC converter has a positive and a negative output lead with saidresonant capacitance joining said leads and a diode clamping thepositive end of said auxiliary switch to said positive output lead. 47.A three stage power source as defined in claim 31 wherein said regulatedDC to DC converter has a positive and a negative output lead with saidresonant capacitance joining said leads and a diode clamping thepositive end of said auxiliary switch to said positive output lead. 48.A three stage power source as defined in claim 32 wherein said thirdstage is a chopper with a power switch having a passive soft switchingcircuit.
 49. A three stage power source as defined in claim 32 whereinsaid given duty cycle is greater than 80% to hold conduction losses ofsaid second stage at a low level.
 50. A three stage power source asdefined in claim 38 wherein said regulated DC to DC converter has apositive and a negative output lead with said resonant capacitancejoining said leads and a diode clamping the positive end of saidauxiliary switch to said positive output lead.
 51. A three stage powersource as defined in claim 38 wherein said third stage is a chopper witha power switch having a passive soft switching circuit.
 52. A threestage power source as defined in claim 38 wherein said given duty cycleis greater than 80% to hold conduction losses of said second stage at alow level.
 53. A three stage power source comprising an input firststage having an AC input and a first fixed DC output signal, a secondstage in the form of an unregulated DC to DC converter having an inputconnected to said first fixed DC output signal, a network of switchesswitched at a high frequency with a given duty cycle to convert saidinput into a first internal AC signal, an isolation transformer with aprimary winding driven by said first internal high frequency AC signaland a secondary winding for creating a second internal AC signal and arectifier to convert said second internal AC signal into a second fixedDC output signal of said second stage, with the magnitude related tosaid duty cycle of said switches, wherein said input first stageincludes a boost converter having a main inductance, a main rectifier,and a power switch having an active soft switching circuit and a thirdstage to convert said second fixed DC output signal of said second stageinto a current suitable for welding, said active soft switching circuitof said input first stage including: an auxiliary switching device and atank circuit closed by said auxiliary switching device, said tankcircuit comprising a resonant inductance coupled in series with saidauxiliary switching device, and a resonant capacitance coupled inparallel with said power switch, said resonant capacitance having afirst resonant capacitor portion coupled in parallel with said resonantinductance and a second resonant capacitor portion coupled in parallelwith said auxiliary switching device.
 54. A power source as defined inclaim 53 wherein said input first stage includes a rectifier and a powerfactor correcting converter.
 55. A power source as defined in claim 53wherein said power switch is in said boost converter.
 56. A power sourceas defined in claim 53 wherein said auxiliary switch is operated inunison with said power switch.
 57. A power source as defined in claim 53wherein said given duty cycle is greater than 80% to hold conductionlosses of said second stage at a low level.
 58. A power source asdefined in claim 54 wherein said power factor correcting converterincludes said boost converter operated by said power switch.
 59. A powersource as defined in claim 54 wherein said auxiliary switch is operatedin unison with said power switch.
 60. A power source as defined in claim54 wherein said given duty cycle is greater than 80% to hold conductionlosses of said second stage at a low level.
 61. A power source asdefined in claim 55 wherein said auxiliary switch is operated in unisonwith said power switch.
 62. A power source as defined in claim 55wherein said given duty cycle is greater than 80% to hold conductionlosses of said second stage at a low level.
 63. A power source asdefined in claim 56 wherein said given duty cycle is greater than 80% tohold conduction losses of said second stage at a low level.
 64. A powersource, as defined in claim 56 wherein said power switch comprises amain boost switch.
 65. A power source as defined in claim 58 whereinsaid auxiliary switch is operated in unison with said power switch. 66.A power source as defined in claim 58 wherein said given duty cycle isgreater than 80% to hold conduction losses of said second stage at a lowlevel.
 67. A power source as defined in claim 59 wherein said given dutycycle is greater than 80% to hold conduction losses of said second stageat a low level.
 68. A power source as defined in claim 61 wherein saidgiven duty cycle is greater than 80% to hold conduction losses of saidsecond stage at a low level.
 69. A power source as defined in claim 64wherein said second resonant capacitor portion is coupled in parallelwith said auxiliary switch by a forward poled diode.
 70. A power sourceas defined in claim 64 wherein said second resonant capacitor portion issubstantially less than ½ the capacitance of said first resonantcapacitor portion.
 71. A power source as defined in claim 65 whereinsaid given duty cycle is greater than 80% to hold conduction losses ofsaid second stage at a low level.
 72. A power source as defined in claim69 wherein said second resonant capacitor portion is substantially lessthan ½ the capacitance of said first resonant capacitance capacitorportion.
 73. A three stage power source comprising an input first stagehaving an AC input and a first fixed DC output signal with a DC to DCconverter including a boost converter having a main inductance, a mainrectifier, and a power switch with an active soft switching circuit, asecond stage in the form of an unregulated DC to DC inverter having aplurality of power switches, said plurality of switches operated by apulse-width-modulated control set at a given duty cycle, and a thirdstage to convert a second fixed DC output of said unregulated inverterinto a current suitable for welding, said active soft switching circuitof said input first stage including an auxiliary switching device and atank circuit closed by said auxiliary switching device, said tankcircuit comprising a resonant inductance coupled in series with saidauxiliary switching device, and a resonant capacitance coupled inparallel with said power switch, said resonant capacitance having afirst resonant capacitor portion coupled in parallel with said resonantinductance and a second resonant capacitor portion coupled in parallelwith said auxiliary switching device.
 74. A power source as defined inclaim 73 wherein said DC to DC converter is said boost converter.
 75. Apower source as defined in claim 73 wherein said DC to DC converter isdriven by a power factor correcting control.
 76. A power source asdefined in claim 73 wherein said given duty cycle of saidpulse-width-modulated control is greater than 80%.
 77. A power source,as defined in claim 73 wherein said power switch comprises a main boostswitch.
 78. A three stage power source as defined in claim 73, whereinsaid pulse-width-modulated control is a phase shiftpulse-width-modulated control.
 79. An active soft switching circuit asdefined in any one of claims 1, 53 and 73, wherein the seriescombination of said resonant inductance and said auxiliary switchingdevice form a first leg of said tank circuit with a first intermediatenode between said resonant inductance and said auxiliary switch, whereinsaid first and second resonant capacitor portions form a second leg ofsaid tank circuit with a second intermediate node between said first andsecond resonant capacitor portions, and wherein said tank circuitfurther comprises a first diode coupled between said first and secondintermediate nodes and a second diode coupled between said secondintermediate node and said cathode of said main rectifier.
 80. A powersource as defined in claim 74 wherein said DC to DC converter is drivenby a power factor correcting control.
 81. A power source as defined inclaim 74 wherein said given duty cycle of said pulse-width-modulatedcontrol is greater than 80%.
 82. A power source as defined in claim 75wherein said given duty cycle of said pulse-width-modulated control isgreater than 80%.
 83. A three stage power source, as defined in claim 75wherein said power switch comprises a main boost switch.
 84. A powersource as defined in claim 76 wherein said duty cycle is greater than90%.
 85. A power source as defined in claim 77 wherein said secondresonant capacitor is coupled in parallel with said auxiliary switch bya forward poled diode.
 86. A power source as defined in claim 77 whereinsaid second resonant capacitor portion is substantially less than ½ thecapacitance of said first resonant capacitor portion.
 87. An active softswitching circuit as defined in claim 79, wherein said first resonantcapacitor portion is larger than said second resonant capacitor portion.88. The active soft switching circuit as defined in claim 79, whereinsaid second resonant capacitor portion controls a rate of increase of avoltage across said auxiliary switching device when said auxiliaryswitching device is turned off.
 89. The active soft switching circuit asdefined in claim 79, wherein said second resonant capacitor portioncontrols a rate of increase of a voltage across said auxiliary switchingdevice when said auxiliary switching device is turned off.
 90. A powersource as defined in claim 80 wherein said given duty cycle of saidpulse-width-modulated control is greater than 80%.
 91. A power source asdefined in claim 81 wherein said duty cycle is greater than 90%.
 92. Apower source as defined in claim 82 wherein said duty cycle is greaterthan 90%.
 93. A power source as defined in claim 83 wherein said secondresonant capacitor portion is coupled in parallel with said auxiliaryswitch by a forward poled diode.
 94. A power source as defined in claim83 wherein said second resonant capacitor portion is substantially lessthan ½ the capacitance of said first resonant capacitor portion.
 95. Apower source as defined in claim 85 wherein said second resonantcapacitance is substantially less than ½ the capacitance of said firstresonant capacitance.
 96. The active soft switching circuit as definedin claim 87, wherein said second resonant capacitor portion controls arate of increase of a voltage across said auxiliary switching devicewhen said auxiliary switching device is turned off.
 97. A power sourceas defined in claim 90 wherein said duty cycle is greater than 90%. 98.A power source as defined in claim 93 wherein said second resonantcapacitor portion is substantially less than ½ the capacitance of saidfirst resonant capacitor portion.